Grinding mills

ABSTRACT

An autogenous grinding mill of large diameter has curved lifters to give improved radially inward displacement of reduced material, after passage through the primary grates; and secondary grates in the form of box sections combined with the curved surfaces of the lifters, for segregation of totally reduced fines, with passage of intermediately sized material back to the mill reduction section.

United States Patent 1 Nelson et al.

[ June 19, 1973 GRINDING MILLS [75] Inventors: John Edward Nelson, Montreal,

Quebec; Rhual Laurent Guerguerian, LaSalle, Quebec, both of Canada [73] Assignee: Dominion Engineering Works,

Limited, Lachine, Quebec, Canada [22] Filed: Apr. 26, 1971 [21] Appl. No.: 137,448

[30] Foreign Application Priority Data May 11, 1970 Canada 82368 [52] US. Cl. 241/70, 241/80 [51] Int. Cl. ..L B02c 17/00, B02c 17/18 [58] Field of Search 241/70, 80

[56] References Cited UNITED STATES PATENTS 1,055,599 3/l9l3 Ball 241/70 X 1,627,487 5/1927 Crowe 241/70 X 1,309,800 7/1919 Greenleaf et al.

3,633,832 l/l972 Fagerholt 241/70 FOREIGN PATENTS OR APPLICATIONS 901,764 ll/l944 France 241/70 665,654 9/1938 Germany 241/70 646,065 5/1937 Germany 241/70 Primary Examiner-Robert L. Spruill Attorney-Raymond A. Eckersley, Frank L.

Neuhauser, Oscar B. Waddell and Joseph B. Forman [57] ABSTRACT An autogenous grinding mill of large diameter has curved lifters to give improved radially inward displacement of reduced material, after passage through the primary grates; and secondary grates in the form of box sections combined with the curved surfaces of the lifters, for segregation of totally reduced fines, with passage of intermediately sized material back to the mill reduction section.

4 Claims, 3 Drawing Figures PATENIEU 9W3 mmniz 7 FIG. 1

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Patent Agent 7 PAINTED-' 3.739.993

SIEH'Z 0F 2 I INVEN'IY JOHN E. NELS RHUAL L GUERGUERIAN BY QOZ M Patent Agent GRINDING MILLS This invention is directed to grinding mills, and in particular to arrangements for controlling the discharge of fine material from mills.

In the use of grinding mills for the reduction of friable material the efficiency of mill operation is dependent upon satisfactory extraction of ground material from the mill. Also with increases being made in the size of individual mills in the interests of improved design and economy, the practical achievement of significant mill size increases is dependent upon satisfactory extraction of the ground medium, to maintain at acceptable levels the power requirements to drive the mill.

Extraction of ground material from previous mills has usually entailed a secondary screening operation outside the mill subsequent to mill reduction, with material of greater than acceptable mesh size being returned to the mill intake end, for reprocessing in the mill, or passed to another mill.

The present invention provides an improved mill extraction arrangement wherein the annular discharge end zone of a mill is provided with inclined lifters to facilitate the discharge of material from the mill.

The present invention provides, in combination with inclined lifters within a mill, an internal screening arrangement to segregate ground material from semifinished material, and to discharge the fine material from the mill, while recycling the semi-finished material within the mill to the mill grinding section, for further reduction.

The present invention is suitable for practicing with both dry and wet mills, and with rod, ball and semiautogenous or autogenous mills.

The present invention thus provides a grinding mill for reducing friable material, having a rotatable shell, at least one material reducing zone therein, loading means at one end of the shell, intermediate partition means within the shell spaced axially from the loading means to form an annular lifter zone adjacent the reducing zone, discharge sizing grate means between the zones to permit passage of grate sized material to the lifter zone from the reducing zone, and a plurality of inclined lifters within the lifter zone, each lifter having a portion of the length thereof extending in inclined non-radial relation inwardly from the lifter zone outer periphery, and material discharge means to discharge from the lifter zone ground material received in operation of the mill from the reducing zone and deposited on rotation of the mill by inclination of the lifters.

The present invention further provides in a grinding mill having a lifter zone spaced axially from a material reducing zone, with a plurality of inclined lifter means extending outwardly to divide the lifter zone into a plurality of like non-radial lifter zone segments, primary grate wall means dividing at least a portion of the lifter zone from the reducing zone to permit passage to the lifter zone of a primary size limited fraction of partially reduced material in operation, box wall means within the lifter zone to subdivide at least some of the lifter zone segments into a lifter fines portion and a lifter return portion, secondary grate wall means forming a portion of the box-wall means to permit a secondary size fraction of the primary size to pass from the return lifter portion to the lifter fines portion, material discharge means connecting with the lifter fines subsegments to receive in operation secondary size material therefrom, and material return means connecting with the lifter return segment portions and the mill reducing zone, to return the portion of the primary size fraction sized in excess of the secondary size back to the reducing zone for further grinding reduction therein.

In order to obtain the maximum benefit of the inclined lifters of the present invention, a form of curved lifter is preferred. However it will be understood that improved lifter operations may be obtained using a straight lifter inclined outwardly from the radial direction to lead in the direction of rotation. A lifter of this inclined form, whether curved, straight or of composite form provides an extended period of material transfer, with a tendency for the material to slide along the supporting surface of the lifter in regulated fashion instead of pitching radially inwardly, as in the case of the radial type of lifter.

The improved operation provided by inclined lifters as contrasted with radial lifters may be at least partially attributed; to the scooping or ploughing effect of the lifter as it enters and passes through the inclined sliding pulp zone; to the increased effective length of lifter surface submerged in the pulp zone, due to the effective non-radial component of lifter length; and to the earlier initial sliding of fines in a discharging direction inwardly from the toe of the lifter, due to the inclination of the lifter beyond the recline angle when located closer to the pulp zone. The time available for fines discharge is correspondingly lengthened, and improved displacement and velocity profile obtained.

Certain embodiments of the present invention are described, reference being made to the accompanying drawings, wherein:

FIG. 1 is a diametral sectional view of a mill accord ing to the present invention; and

FIG. 2 is a pictorial representation of the mill lifter zone viewed from within the mill, with FIG. 3 comprising an enlargement of a portion of one lifter.

Referring first to FIG. 1, the mill shell 10, shown in diametral section, is mounted for rotation about a horizontal axis, having hollow trommels at the shell ends ll, 12 for charging and discharging the mill, in known fashion.

The inclined shell walls 15, 16 are provided with wedge bars 17 serving to energize or entrain the semifluid contents of the mill, and are rubber lined to resist abrasion.

At the mill discharge side an annular partition 20 segregates the reducing or grinding zone 21 from the lifter zone 22. A plurality of primary grates 23, 25, illustrated also in FIG. 2 in the partition 20 permit the passage from the reducing zone 21 to the lifter zone 22 ofsuitably reduced material, including portions reduced to the desired minimum mesh size and below, together with larger particles of intermediate size.

Referring more particularly to FIG. 2, the lifters 30 subdivide the annular lifter zone 22 into a plurality of lifter sub zones or segments 33. Within each of the seg merits 33 in the illustrated embodiment is a so called rubber box 35 having an upper surface thereof in the form of a secondary grate 36. The wedge bars 17 are omitted, for purposes of clarity.

The inwardly extending boxes 35 which extend for the full axial extent of the lifters 30 at the radially outer extremity, are of reduced axial width and increased pcripheral extent at the mouths 37, to form an annular discharge of abutting slots, leaving free a portion of the lifter spaces 33 axially adjacent thereto, to permit the passage of coarser material too large to pass through the secondary grates 36.

By means of suitable trommel partition means the material of secondary size and smaller passes axially through the trommel 12 and is collected outside of the mill 10, for subsequent use. The material of primary size and smaller is returned by the trommel partition means into the mill for further reduction.

While the composite lifters including boxes 35 are of curved double wall construction, it will be understood that other forms of inclined lifter may be used.

The boxes 35 are lined with or formed of suitable rubber, plastic or other wear resistant material.

In operation, the earlier discharge of fines after leaving the pulp pool leads to earlier discharge from the lifter zone. This effectively reduces the velocity of discharge, to minimize or prevent throw of the material against the back of the preceding lifter. More importantly the work done against the mill is reduced, owing to the reduced lift head, so that power requirements diminish, or mill size or loading may be correspondingly increased.

While the present invention has been disclosed in relation to a mill having a single reduction zone, the functioning of a multi-zone mill may also be enhanced.

The use of non-radial straight lifters is contemplated, arranged in semi-chordal relation to the grinding zone.

What I claim as new and desire to secure by Letters Patent of the United States is:

l. A grinding mill for reducing friable material to a pulp condition, having a rotatable shell, at least one material reducing zone therein, loading means at one end of the shellto supply said material to said zone, intermediate partition means within the shell spaced axially from the shell other end to form an annular lifter zone axially adjacent the reducing zone, first sizing grate means between the zones to permit passage of grate sized material to the lifter zone from the reducing zone, a plurality of inclined lifters within the lifter zone each having a portion of the length thereof extending in inclined non-radial relation inwardly from the lifter outer zone periphery, to provide advanced initial inward acceleration of said material on inclination of the lifter, said lifters dividing said lifter zone into adjacent primary zone portions, second grate means in the surface of said lifters, wherein the improvement comprises box means underlying said second grate means to receive in operation a secondary fraction of material passing through the second grate means, said box means projecting within the adjacent said primary zone portion in substantially non-impeding material flow relation therein, and first and second material discharge means connected respectively to said primary zone portions and said box means to convey the respective contents thereof after radially inward passage therethrough to axially separated planes of discharge.

2. The mill as claimed in claim 1 wherein said second grate means is spaced inwardly of the radially outer ends of said lifters to locate said underlying box means radially inwardly of an annular outer portion of said first sizing grate means and provide said first sizing grate an outer annular zone of substantially unrestricted grate area.

3. The mill as claimed in claim 1 wherein said second discharge means connected to said box means comprises an annular discharge port having adjacent segment portions thereof connected to respective ones of said box means to pass said secondary material fraction axially outwardly of the mill.

4. The mill as claimed in claim 1 wherein said first discharge means connected to said primary segments comprises an annular discharge port having adjacent segment portion thereof connected to respective ones of said primary segments, to pass said primary pulp portion axially into the mill. 

1. A grinding mill for reducing friable material to a pulp condition, having a rotatable shell, at least one material reducing zone therein, loading means at one end of the shell to supply said material to said zone, intermediate partition means within the shell spaced axially from the shell other end to form an annular lifter zone axially adjacent the reducing zone, first sizing grate means between the zones to permit passage of grate sized material to the lifter zone from the reducing zone, a plurality of inclined lifters within the lifter zone each having a portion of the length thereof extending in inclined non-radial relation inwardly from the lifter outer zone periphery, to provide advanced initial inward acceleration of said material on inclination of the lifter, said lifters dividing said lifter zone into adjacent primary zone portions, second grate means in the surface of said lifters, wherein the improvement comprises box means underlying said second grate means to receive in operation a secondary fraction of material passing through the second grate means, said box means projecting within the adjacent said primary zone portion in substantially non-impeding material flow relation therein, and first and second material discharge means connected respectively to said primary zone portions and said box means to convey the respective contents thereof after radially inward passage therethrough to axially separated planes of discharge.
 2. The mill as claimed in claim 1 wherein said second grate means is spaced inwardly of the radially outer ends of said lifters to locate said underlying box means radially inwardly of an annular outer portion of said first sizing grate means and provide said first sizing grate an outer annular zone of substantially unrestricted grate area.
 3. The mill as claimed in claim 1 wherein said second discharge means connected to said box means comprises an annular discharge port having adjacent segment portions thereof connected to respective ones of said box means to pass said secondary material fraction axially outwardly of the mill.
 4. The mill as claimed in claim 1 wherein said first discharge means connected to said primary segments comprises an annular discharge port having adjacent segment portion thereof connected to respective ones of said primary segments, to pass said primary pulp portion axially into the mill. 